Bob's Logistics adds a setting to increase the Storage Capacity of Storage Tanks.

This setting is '25' by default so Tanks have 25,000 Fluid Capacity and is called 'bobmods-logistics-storagetankbase'
Valid from 10 to 1000

I'd like to increase the Big Storage Tank by this value (Divided by 25)
So if the player sets 50 (50k Storage Tanks), Big Storage Tank would hold 2 million.

so its like a multiplier?

Yes. Allowing the big tank to scale with bobs modifier.

As it stands, your tank becomes useless when bobs has a large multiplier. Because you could then have 2 million in the size of a vanilla tank.

ok now i get it